The aim of this study was to examine the effects of the plant signalling hormone jasmonic acid (JA) on the induced resistant mechanisms of Lycopersicon esculentum , or common tomato plants, when the seeds were treated with a 1.5mM solution of JA, most notably the effect on polyphenol oxidase (PPO), a proteinase inhibitor in plants. Polyphenol oxisase, an oxidative enzyme has been found to act as a protease inhibitor in the guts of foraging insect herbivores (Ryan et al., 1990) and have also been found to be commonly used in plants as marker genes to the induced wound response.
In their natural environment, damage is inflicted upon plants by many means such as drought effects, the effects of increased salinity, but most notably by herbivore grazing. To combat the grazing of herbivores, plants have evolved unique mechanisms to reduce the damage inflicted upon them. Not only do plants need to combat the direct effects of herbivore grazing, but also the indirect effects. For example, microbial pathogens such as Agrobacterium tumefaciens which causes crown gall disease, may enter at and attack the wound site, which has resulted in the evolution of a complex system of defence against both the herbivores and pathogens. The responses are almost always induced responses and the response pathways are directly activated as a result of herbivore wounding.
Figure 1a. (Roberts M. R., et al., 2001) Showing a basic scheme for a plant response to wounding. Mechanical wounding, simulating herbivore grazing elicits the expression of local and systemic genes. Shown are the general events, from wounding which causes a calcium ion flux to occur, inducing the expression of genes which from wound healing proteins and the synthesis of signalling hormones, which in turn leads to defence gene expression and then the expression of defence molecules. Also shown is the systemic signalling pathway where the wounding induces signals which cause defence genes to be expressed in unwounded leaves.
Jasmonic acid, the inducement molecule used in this study was first identified as a signaling molecule in induced resistance to wounding when studies lead to the identification of methyl jasmonate (MeJA) as a strong inducer of proteinase inhibitor (Pin) genes in tomato plants. JA has been found to be present in many plant species and is involved in regulating diverse plant functions including plant resistance and senescence (Creelman et al., 1997) Studies have shown that although MeJA is not elicited in sufficient quantities or over sufficient distanced to induce Pin genes in neighbouring plants, JA does act as a systemic signaling molecule, inducing the Pin genes elsewhere in an unwounded area of the plant (Farmer et al., 1992). Jasmonic acid biosynthesis is catalysed by a process called the ocatdecanoid pathway, catalysation occurring via several different enzymes which are regulated by compartmentalization in different sub-cellular vacuoles.
JA synthesis begins with the release of lipid precursor molecules from cellular membranes such as the plasma membrane and the chloroplast membranes, but as of yet the membrane types have not been fully identified. Phospholipase D (PLD) has recently been shown to be essential for JA synthesis (Wang et al., 2000) and may produce substrates for phospolipase A2 which activates lipoxygenases or may directly activate them. Lipoxygenases are involved in the next stage of JA synthesis, which probably occirs in the chloroplast.
The lipoxygenases convert linolenic acid into lipid hydroperoxides which are then converted into the key intermediate 12-oxo-phytodeienoic acid (OPDA) through the action of allene oxide synthase and allene oxide cyclase (AOS and AOC). In the cytoplasm, OPDA undergoes reduction, and then is subjected to 3 -oxidations in the peroxisome to produce the Jasmonic acid product. In addition to the synthesis of MeJA, Krumm et al., (1995) and Wasternack et al., (1998) have shown that there are other jasmonate-related molecules formed such as bioactive amino acid conjugates.
Figure 1b. A model of the octadecanoid signaling pathway, culminating in the synthesis of MeJA which then induces the plant defences and synthesis of PPOs and other volatiles.
JA signaling in plants is not just limited to the accumulation of high levels in wounded leaves as increases of JA in unwounded leaves have also been observed, although not to the same extent (Bowles, 1998). It is known that JA is a requirement for most systemic wound responses, it is not known if it has specific functions in the systemic leaves or only acts in the wounded leaf to mediate systemic signaling (Roberts et al., 2001, Bowles., 1998). The site of JA synthesis in the plant seems to be instrumental in inducing an appropriate response to herbivore wounding. Farmer and Ryan (1990) and Farmer et al (1992) discovered that the appliance of MeJA and JA exogenously to plants causes the inducement of defence gene expression and can provide resistance to certain insects. Foliar JA application has been shown to increase levels of PPO and induced plants have shown 60% less leaf damage than control plants (Thaler, 1999).
The exogenous application of JA onto foliage acts as a resistance inducer in some crop plants, other studies have shown that treatment of seeds before crop planting can cause induced systemic resistance (ISR) in certain plants. Shailasree et al., (2001) studied the effects of -aminobutyric acid (BABA) on pearl millet and how the treatment of seeds before seeding caused ISR against downy mildew disease in the seeds themselves and plants germinated from those treated seeds.. Plants raised from seeds treated with the inducer showed a disease incidence of 10 and 12% compared to 71 and 76% in the control plants. This suggests that induced resistance in seeds treated with BABA remained operative through the vegetative and reproductive growth of the plants. If ISR can be initiated through seed treatment in these experiments, then it could also be possible in tomato seeds/plants when treated with JA.